A typical type of valve drive apparatus used for a double overhead camshaft engine has a pair of valve trains for one cylinder of the engine. The pair of valve trains, having the same structural elements, are symmetrically arranged in a V-formation with a predetermined relative angle with respect to a center axis of the cylinder. Although camshafts for timely, or sequentially, opening and closing valves through the valve trains are driven mechanically or geometrically in the same direction, they are driven dynamically in the opposite directions with respect to rocker arms. That is, because the geometric relative arrangement of the rocker arm with respect to the direction of rotation of the camshaft is opposite between the pair of valve trains and because leverage of the rocker arm changes with the rotation of camlobe, characteristic curves representing the changes of leverage of the rocker arms are symmetrical with respect to rotated angles of the camlobe. Accordingly, at a moment immediately before the valve closes, the leverage of either one of the rocker arms is larger than that of the other.
As is well known in the art, unusual vibration of the valve train is one of the primary factors that adversely affects an engine and tends to prevent satisfactory operation at high speeds. That is, when a harmonic component, having a low frequency, of a valve lift curve of a valve becomes close to a natural frequency of the valve train as engine speed increases, the natural frequency of the valve train becomes too high, so that the valve train causes unusual vibration, such as jumping or bounding. In order to raise a critical engine speed at which the engine causes unusual vibration, it si absolutely necessary to increase the natural frequency of the valve train. Both an equivalent rigidity of valve train, which depends greatly on the leverage of rocker arm, and an equivalent mass of the valve are the primary factors in determining the natural frequency of the valve train. Since the equivalent rigidity of valve train becomes higher as the leverage of the rocker arm becomes smaller, the use of a small leverage rocker arm is favorable and preferred in order to allow the engine to increase the critical engine speed in a satisfactory manner. Additional details may be obtained by reference to a book entitled, "Automobile Engineering Handbook: IV," pages 1-52, published by Automobile Technology Association.